1. Field
The present disclosures herein relate to aircraft engines, and more specifically to systems and techniques for augmenting the power capability of aircraft engines in high altitude environments.
2. Background
Ongoing development and growth in the area of onboard aircraft electrical systems and electronic sub-systems has resulted in a desire to augment existing aircraft systems with supplemental electrical generating capability. However, standard aircraft production-design characteristics generally leave little, if any, room for significant electrical or cooling air systems expansion or modification. Thus, it is difficult for aircraft to accommodate post-production systems additions. The traditional approach of original equipment manufacturers with respect to expanding on-board power and electrical generating capability usually leads to extensive and costly aircraft and/or engine modifications.
Aircraft are often powered by gas turbine engines, which have a great power-to-weight ratio compared to internal combustion reciprocating engines. Gas turbine engines are commonly considered to be “over-powered” at low altitudes, because of their high power-to-weight ratio. However, at high altitude, when the air gets thinner, air-breathing internal combustion engines lose power. Even gas turbine engines can quickly become “under-powered” as an aircraft ascends. Unfortunately, power enhancement modifications to an aircraft engine often require costly structural alterations to the airframe itself. Thus, in addition to the main engines, aircraft often utilize additional small gas turbine engines that may be installed within the aircraft. These additional engines may generate electric power and provide pressurized air for power requirements while the aircraft is on the ground. Generally, these devices have their functions taken over in flight by the main engine. However, as electrical requirements for passenger amenities and other electronic needs have increased, these auxiliary power units have become correspondingly larger. In modern aircraft, auxiliary power units are often utilized in-flight. Although many auxiliary engines are now overpowered at sea level, they generally are only able to provide constant power up to altitudes of about 25,000 ft. (“FL25”), and have diminishing power as the increases beyond that. Gas turbine engines cannot easily be made any larger, as the increase in size and weight would require significant structural modification to the airframe itself.
In short, modern aircraft including military aircraft, which have high requirements for electrical power, suffer deficiencies when equipped with gas turbine engines, because they lose power at high altitude but cannot compensate with increased size due to airframe structural limitations. Thus, the in-flight power generating capability of aircraft is often significantly limited under prior art constructs. One result is that there is not currently a gas turbine power system capable of operating at high altitude with the ability to maintain the increasing demand for more horsepower to drive a generator and produce cooling air in sufficient quantity, without requiring significant modification to airframe structures.